function Person(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  
  const lydia = new Person("Lydia", "Hallie");
    
    const sarah = Person("Sarah", "Smith"); // this是在全局下执行，会在全局挂载
  
  console.log(lydia);
  console.log(sarah);
  console.log(this.firstName);